In a medium bowl, combine olive oil, lime juice, garlic, brown sugar substitute, chili powder, cumin, red pepper flakes, salt, smoked paprika, oregano, and cilantro leaves. Whisk to blend.

Reserve half of the marinade for later use. Add the beef strips to the remaining marinade and toss to coat. Let sit while you prepare the vegetables.

Heat one large skillet over medium-high heat. Add the beef strips and cook, stirring occasionally, for 5-7 minutes or until no longer pink in the center. Remove the beef and tent with foil to keep warm.

In a second large skillet, heat olive oil over medium heat. Add the cauliflower rice and season with salt and pepper. Cook for 10-15 minutes, stirring frequently, until tender.

After removing the beef, add the sweet peppers, onions, and 2-3 tablespoons of the reserved marinade to the same skillet. Cook for 5-7 minutes, until the vegetables are charred and fork-tender.

Divide the cauliflower rice evenly among four bowls. Top with equal portions of beef strips and fajita vegetables. Garnish with avocado slices, fresh cilantro, and drizzle with the reserved marinade, if desired.